This invention relates to a multilevel modulator capable of producing a composite modulated signal in response to a main data signal and a subdata signal and to a multilevel demodulator for use as a counterpart of the multilevel modulator.
A multilevel modulating system of the type described is exemplified in U.S. Pat. No. 4,498,050 issued Feb. 5, 1985, to Yasuharu Yoshida, the instant applicant, and assigned to NEC Corporation. This system is a composite PSK-PSK modulating system which comprises a 2.sup.k -phase PSK modulated component carrying the main data signal and a 2-phase PSK modulated component carrying the subdata signal where k represents a positive integer. In the system, the main data signal has a first bit rate f.sub.1 which is not less than a second bit rate f.sub.2 of the subdata signal. As a result, the subdata signal can be effectively transmitted without adversely affecting a bit error rate of the main data signal.
In the meanwhile, a multilevel quadrature amplitude modulating system has recently become the center of a digital transmission system. In the multilevel quadrature amplitude modulating system, a pair of quadrature-phase carrier signals are amplitude modulated by first and second input signals into a multilevel quadrature amplitude modulated signal. When each of the first and the second input signals is an n-bit binary signal where n is a positive integer which is not less than two, the multilevel quadrature amplitude modulated signal has 2.sup.2n output signal points on a phase plane which has an origin and real and imaginary axes orthogonally crossing at the origin. Such the multilevel quadrature amplitude modulator is exemplified in U.S. patent application Ser. No. 779,217 filed Sept. 23, 1985, by Junichi Uchibori et al, for assignment to NEC Corporation.
The multilevel quadrature amplitude modulator can effectively utilize a carrier level. Therefore, a large-capacity digital radio communication network ordinarily comprises the multilevel quadrature amplitude modulator.
By the way, the conventional composite modulating system is restricted to the PSK modulation. Namely, the conventional composite modulating system can not be applied to the multilevel quadrature amplitude modulating system.